Currently, many wireless radios use frequency division duplex (FDD) or time division duplex (TDD) to avoid interference between the transmitted signal and the received signal. In cellular communication, the sensitivity to interference is aggravated by the fact that the transmit power is many orders of magnitude larger than the received signal strength, making the receiver susceptible to co-located transmitter noise and signal leaking into the receiver front end.
Digitally generated cancellation systems create an inverted duplicate of the baseband deterministic signal appearing at the transmit baseband digital interface and up convert this signal to radio frequency (RF) to provide “active” signal cancellation. However, this method does not adequately address the transmitter broadband noise or non-linearities created by downstream RF analog transmit components leaking into the receiver.
Modern cellular base station radios, WiFi, and other wireless systems employ a technology commonly referred to as MIMO (multiple-input, multiple-output), where multiple transmitters and/or receivers operating at the same frequency increase the capacity of the link. This system exacerbates the leak problem since multiple transmitters are leaking into any given receiver. This leak problem is further aggravated by the fact that newer generation cellular technologies (e.g., 5G) employ division free duplexing in which the transmitter transmits at the same time and on the same frequency that the receiver receives signals.